1. Field of the Invention
The present invention relates to a fuse trimming circuit having a fuse element formed on a semiconductor substrate in an integrated circuit.
2. Description of the Related Art
A conventional fuse trimming circuit using a fuse element formed on a semiconductor substrate is disclosed by such patent documents as U.S. Pat. No. 4,532,607 (D1), U.S. Pat. No. 5,731,733 (D2), and Hatsumei Kyokai Kokai Giho, No. 2001-6215 (D3).
FIG. 1 is a circuit diagram showing a conventional fuse trimming circuit disclosed by the document D1. The fuse trimming circuit is provided with P channel MOS transistors (PMOS's) 1a, 1b, a resistance detection part 1 having a resistive element 1c and a fuse element 1d, a PMOS 2a, a PMOS 2b, and an amplifier 2 having N channel MOS transistors (NMOS's) 2c, 2d, 2e, and 2f. 
In the resistance detection part 1, a voltage difference corresponding to a resistance difference between the resistive element 1c and the fuse element 1d appears across nodes Na and Nb. The voltage difference across the nodes Na and Nb is amplified in the amplifier 2 and then generated from a node Nd. When the fuse element 1d is cut or blown, a resistance of the resistive element 1c (R1c) becomes smaller than that of the fuse element 1d (R1d) and thus a voltage at a node Na (VNa) becomes smaller than that at a node Nb (VNb), so that a supplied voltage at the Nd decreases through the amplifier 2. Therefore, a low level signal having a low(L) level is generated from an output terminal ZOUT.
When the fuse element 1d is kept on un-blown and the R1c is made larger than the R1d and thus the VNa is larger than the VNb, so that, a high level signal having a high (H) level is generated from the output terminal ZOUT. Even when the fuse element 1d is not completely disconnected or blown state, the circuit can be controlled by fuse information representing a blown state or an un-blown state of the fuse element generated from the output terminal ZOUT as long as the R1c is smaller than the R1d. The fuse information representing a blown state or an un-blown state of the fuse element is used for operating the circuit by activating the amplifier 2 with the signal “Enable” when an IC is switched on. The semiconductor integrated circuit, therefore, prevents from malfunctioning even when the fuse element is not completely disconnected due to the incomplete disconnection.
The documents D1 and D2 respectively disclose fuse trimming circuit, each describing such a technique that detects a blown state or an un-blown state of a fuse element by comparing currents flowing through the fuse element and a resistive element which are connected in parallel with each other and latches the determination result.
However, the conventional fuse trimming circuits disclosed by the documents D1 to D3 encounter the following problems:
In the circuit shown in FIG. 1, the circuit normally operates as long as the resistance of R1c is smaller than the resistance of R1d even when the fuse element 1d is in an incomplete brown state. The PMOS 1b and the fuse element Id are connected in series to each other between a power supply (VDD) node and a ground (GND) node, so that a current continues to flow through the fuse element 1d through the PMOS 1d in an un-blown. When the current continues to flow through the fuse element 1d in an incomplete blown state, the fuse element gradually connects again, thus causing a risk that a resistance R1d of fuse element 1d gradually decreases. A problem relating to reliability of a circuit arises that malfunction occurs in a circuit that operates normally if the circuit is used for a long time period
The conventional fuse trimming circuits are not suitable for low-power consumption type large-scale integration (LSI) circuit because a stationary current keeps flowing through the resistive element 1c and the fuse element 1d in the incomplete blown state even if the enable signal “Enable” is changed to the level “L” and the NMOS 2e is switched off.